Thermal desorption tube collection system and method

ABSTRACT

A thermal desorption tube collection system uses a thermoelectric cooler to collect and concentrate gas samples. In some modes, the operation of the cooler is reversed to flow the concentrated sample directly into a separator such as a gas chromatography system. Components resolved in time by a thermal desorption separator accumulate in a sample cell and are analyzed by electromagnetic radiation-based spectroscopic techniques. Also presented are methods for analyzing biogas samples.

RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Application No. 62/636,628, filed on Feb. 28, 2018 and U.S.Provisional Application No. 62/636,623, filed on Feb. 28, 2018. All ofthe afore-mentioned applications are incorporated herein by thisreference in their entirety.

BACKGROUND OF THE INVENTION

Gas Chromatography (GC) is used to resolve a mixture into its variouscomponents according to retention profiles of the different moleculespassing through the GC column.

While the technique can separate mixtures containing hundreds ofsubstances, identifying the molecules that elute from the column is moreproblematic. To address the need for rapid and sensitive identificationof the molecular species present, GC has been integrated with techniquessuch as mass spectrometry (MS) or Fourier transform infrared (FTIR)spectrometry.

Gas chromatography-mass spectrometry (GC-MS) is probably the mostwidespread tandem technique in the analytical instrumentation industrytoday. GC-MS gas analysis systems are versatile and are employed acrossmany different industries, particularly for environmental, chemical,petroleum, pharmaceutical, and toxicological applications.

While GC-MS is a fast, sensitive technique suitable for multiplecomponent detection and spectral identification, capable of measuringatomic species and supported by large available spectral libraries, itsuffers from many disadvantages. These include compound separation toprevent MS interferences, non-linear calibrations, poor precision andaccuracy (requiring constant calibration) and limited dynamic range.Problems also are encountered when high concentrations are present thatcan allow for chemical ionization to occur, generating questionabledata.

While GC-MS is the more commonly deployed solution, GasChromatography-Fourier Transform Infrared Spectrometry (GC-FTIR)provides a powerful gas analysis system that is particularly useful todistinguish among structural isomers that have identical electron impactand chemical ionization mass spectra.

Nevertheless, historically the designs of GC-FTIR systems have beenplagued with their own limitations. For example, many GC-FTIR samplecells utilize a “light pipe” (typically a cell or cuvette used forpassing both gas eluted from the GC column, and light from the FTIRinterferometer). The light pipe is made relatively short to prevent peakdilution through the IR cell and its eventual IR detection or secondarydetection. Since IR absorption is proportional to cell path length, thisshort path length limits the sensitivity (minimum detection limit (MDL))of the technique. Problems also arise in cases in which GC peaks comeoff very quickly. Since the light pipe has a relatively large volumewhen compared to the flow rates of the GC, the gas can become diluted,making measurements more difficult.

More recently, GC-FTIR techniques and systems were disclosed in U.S.Patent Application Publication No. 2015/0260695 A1, with the titleProcess and System for Rapid Sample Analysis, published on Sep. 17,2015, now U.S. Pat. No. 9,606,088, both documents being incorporatedherein by this reference in their entirety. The general objective was tocouple existing or newly developed systems, such as GCs, and/or opticalspectroscopy systems, such as FTIRs, in ways that reduce or minimize thedeficiencies encountered with conventional arrangements. In general, thesample was directed from a temporally-resolving separator to a samplecell, e.g., a gas cell that fully or partially integrates the componentsprovided by the separator. Fluids, e.g., gas(es), were allowed toaccumulate in the sample cell, effectively integrating their spectralsignatures. Multiple spectra obtained over a time interval could then beaveraged to best measure the integrated concentration in the samplecell. Obtaining a moving background that includes spectra from apreviously eluted sample component, e.g., previously eluted chemicalspecies, allowed for the analysis of the current eluting componentswithout interference from previously eluted components. The integratedand averaged multiple spectra were corrected by using a similarlycollected moving background, and the corrected data are compared toknown spectra to identify one or more components, e.g., chemical speciessuch as atoms, molecules, molecular fragments, ions, present in thesample component.

Analytical thermal desorption was adapted from injection procedures forGC's. It can be used in GC-MS or GC-FTIR systems, as well as other gasanalysis systems. Injector liners were packed with sorbent that wouldabsorb organic compounds in a sample gas. The sorbent was then insertedinto the inlet of the GC's. This was common in occupational monitoring.

A more modern solution involves thermal desorption tubes (TDTs). Theseare sold commercially and, in most cases, are disposable. They aretypically standardized glass tubes that are preloaded with the sorbent.Metal tubes also can be employed.

In thermal desorption tubes, volatiles are collected on the sorbent asthe sample gas is passed through the tube. One popular sorbent forthermal desorption is poly(2,6-diphenyl-p-phenylene oxide). Then, thetube is connected to a GC where it is heated to release those volatiles.

When the sorbent is then heated in a flow of gas, the captured compoundsare released and concentrated into a smaller volume. They then flow intothe GC. Generally, these systems can be divided into single-stagesystems and two-stage systems. In single-stage systems volatiles arecollected on the sorbent tube, are then released by heating the tube ina flow of gas, and then flown directly into the GC. In contrast, intwo-stage systems the gas stream from the sorbent tube is collected on asmaller tube integral with the TDT. This is also called a focusing trapor cold trap. Heating the trap releases the volatiles once again but ina smaller volume. This improves sensitivity and provides better GCperformance. Typically, the focusing trap is held at or below roomtemperature. However, higher temperatures are sometimes used to reducethe amount of water inside the trap.

SUMMARY OF THE INVENTION

The present system and method concerns thermal desorption tube systemsand electronic control if their temperatures. This system cools and/orheats the tubes with a thermoelectric cooler.

In a preferred embodiment, the system could be a standalone sampler thatcould be used optionally for hot and/or cold sampling. For example, insome embodiments, it further includes a battery so that even a powerconnection is not required. Cold sampling would be used for veryvolatile materials and possibly hot for semi volatiles to non-volatilecompounds. Techniques described herein can also find application indetecting non-volatile, often very non-volatile, or low semi-volatileimpurities in a gas. In one implementation, oil contamination (tracepump or compressor oil) present in a biogas sample, for example, isdetected and, in many cases, quantified.

The system could be deployed as the front end of a GC as discussed indetail below. On the other hand, the system could be used as both acollector and desorber potentially. In some cases, the system might evendesorb directly into the sample cell of the FTIR or other form ofspectrometer. In some modes, the operation of the cooler is reversed tothereby heat the TDT and thus flow the concentrated sample directly intoa separator such as a gas chromatography system.

Practicing the invention can have many advantages. For example, coolingthe TDT increases the amount of material that can be collected. Samplescan be concentrated, resulting in lower method detection limits (MDLs).By relying, as shown herein, on a Peltier thermoelectric device, the TDTcan be both heated and cooled, to accomplish both collection anddesorption in a simple and effective approach.

By using a cooled TDT, specialized TDTs are not as important. In thisrespect, it was possible to demonstrate the ability of collectingformaldehyde and other gases (typically vapors) not normally stored onthe traditional TDT typically used for VOCs. Employing a heated TDT maypotentially allow the collection of very large samples containing semivolatile materials. It might also make possible a faster samplecollection process, with a higher flow rate through the TDT.

Some of the embodiments described herein relate to standalone TDT-coolersystems that can be battery operated and thus are portable andindependent of finding an available power source. With a cooled TDT,standalone versions of the system described herein make possible thecollection of larger samples than could be collected with an ordinary,uncooled TDT. An important application of a standalone device such asdescribed herein relates to the collection and recovery of light orvolatile species.

Embodiments that employ two channels make possible switching betweencollection and desorption and is particularly useful for onlinesampler/desorber systems.

Some sample analyses can be conducted without a GC separator (column),advantageously streamlining the equipment and methods in someembodiments, for example. Thus, some of the arrangements describedherein do not require sample splitting, a common practice when using GCseparators. Also not needed is a secondary trap, e.g., a cold or cryotrap, that is sometimes used to “focus” or further concentrateimpurities in a sample gas.

In general, according to one aspect, the invention features a thermaldesorption tube collection system comprising at least one thermaldesorption tube and a thermoelectric cooler for cooling and/or heatingthe at least one thermal desorption tube.

In some cases, it could comprise two or more thermal desorption tubes,however. Each of the two thermal desorption tubes is connected to amanifold and/or a vacuum pump. Additionally, a flow through each of thetwo thermal desorption tubes is controlled by a mass flow controller ora rotometer.

In one possible mode of operation, a first thermal desorption tube isconfigured to operate in a sample collecting mode while a second thermaldesorption tube is configured to operate in a sample analysis mode, andthen they function could be reversed. So, in one case, a first thermaldesorption tube is cooled by the thermoelectric cooler, while a secondthermal desorption tube is heated by the same thermoelectric cooler.

In general, according to another aspect, the invention features astandalone thermal desorption tube collection system comprising at leastone thermal desorption tube, a thermoelectric cooler for cooling and/orheating the at least one thermal desorption tube, and a controllerpowered by a battery.

In general, according to another aspect, the invention features a sampleanalysis system comprising two thermal desorption tubes configured for asample collection and sample analysis ping pong arrangement, athermoelectric cooler for cooling a sample collection thermal desorptiontube while heating a sample analysis thermal desorption tube, a gaschromatography system for separating a vapor desorbed from the heatedthermal desorption tube into components, and a sample cell for receivingthe separated components from the gas chromatography system andelectromagnetic radiation for a spectrometric analysis of compounds inthe sample cell.

In examples, the spectrometric analysis is FTIR spectrometry. On otherexamples, a tunable laser spectrometer could be used.

In some cases, an input director system is provided for selectivelycoupling inputs of the thermal desorption tubes to either a gas to beanalyzed or a carrier gas or a vent. An output director system canselectively couple outputs from the thermal desorption tubes to eitherthe gas chromatography system or a vacuum pump or a vent.

In general, according to another aspect, the invention features a methodfor collecting a sample, comprising collecting a gas or vapor on one ormore thermal desorption tube that is cooled by a thermoelectric cooler.

In general, according to another aspect, the invention features a methodfor collecting a sample, the method comprising selectively directingeither a sample gas or a carrier gas to inputs of two thermal desorptiontubes. The two thermal desorption tubes are cooled or heated by a pingpong temperature control system comprising a thermoelectric cooler.

In general, according to another aspect, the invention features a sampleanalysis method or system, comprising collecting oil present in abiomethane gas sample in a sample collection device, desorbing collectedoil in the sample collection device, directing oil vapors to a samplecell, and obtaining a spectral response of the oil vapors in the samplecell using spectrometer, such as an FTIR analyzer.

The above and other features of the invention including various detailsof construction and combinations of parts, and other advantages, willnow be more particularly described with reference to the accompanyingdrawings and pointed out in the claims. It will be understood that theparticular method and device embodying the invention are shown by way ofillustration and not as a limitation of the invention. The principlesand features of this invention may be employed in various and numerousembodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a perspective view of a TDT collection system of the presentinvention;

FIG. 2 is a schematic diagram of a GC-FTIR sample analysis system fortesting samples collected in the TDTs and/or TDT collection system;

FIG. 3 is a schematic perspective view of a TDT collection system foroccupational and home air testing;

FIG. 4 is a schematic cross-section view of a dual TDT collection systemfor a GC-FTIR sample analysis system for ping-pong operation;

FIG. 5 is a schematic diagram of a flow control arrangement foroperating the sample collecting TDTs;

FIG. 6 is a plot of absorbance as a function of wavenumber obtained bythe direct vaporization of oil in a N2 with an MDL of about 20 ng;

FIG. 7 presents a plot of absorbance as a function of wavenumber for anoil versus dodecane calibration;

FIG. 8 through 12 are plots of absorbance as a function of wavenumberspectra of a sample oil using various hydrocarbons calibrations, with205,000 ng oil being expected; and

FIG. 13 is a calibration curve of dodecane in amounts from 0 to 800,000ng.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a TDT (thermal desorption tube) collection system 100,which has been constructed according to the principles of the presentinvention.

In more detail, gas, such as hot exhaust gas from a chemical processingsystem or energy generation system, or gas from occupational monitoringis directed into a gas inlet 110. It then flows through a sample gastube 118 to a gas outlet 112.

In the illustrated embodiment, the sample gas tube 118 is insulated fromthe surrounding environment in order to ensure that the gas streammaintains its temperature as it flows through the sample gas tube 118.Specifically, a foam outer covering, thermal insulator, 180 is shown inthe specific illustrated embodiment. This is held in place by four zipties 182 so that it can be easily removed and re-secured to the samplegas tube 118.

In other embodiments, rather than a foam covering, other insulationapproaches could be used to ensure that the gas stream does not decreasein temperature as it flows through the TDT collection system 100. Suchapproaches include, for example, insulated fiber wraps.

In still other embodiments heater tape or other resistive heating system140 is affixed to the gas tube 118, under the thermal insulator 180. Theresistive heating system 140 is controlled by the controller 11 tomaintain a prescribed temperature. Preferably feedback control isemployed by placing a gas temperature sensor 142 in the tube 118,upstream of the tube 118, and/or downstream of the tube 118. The gastemperature sensor 142 detects the temperature of the gas flowingthrough the tube 118 and the sensor 142 is monitored by the controller11. The controller 11 then drives the resistive heating system 140 tomaintain a prescribed temperature for the gas flowing through the tube118.

The sample gas tube 118 forms part of a manifold. Specifically, there isa first sample port 120 and a second sample port 122 through which gassamples can be taken from the gas stream flowing through the tube 118.

In the illustrated embodiment, the first sample port 120 connects to afirst sample control valve 124. In a similar vein, the second sampleport 122 connects to a second sample control valve 126.

In some embodiments, the control valves 124, 126 are manually operated.This is shown in the illustrated embodiment, where there are valvehandles connected to the valve stems of each of the control valves 124,126.

Nevertheless, in another embodiment, the first sample control valve 124and the second sample control valve 126 are also under the control of acontroller 11 and include electrically controlled mechanical actuators.Specifically, the controller 11 selectively opens and closes the firstsample control valve and second sample control valve 124, 126 undercomputer control to control the timing of sampling and the length of thesampling.

After each of the sample control valves 124, 126 are respective topfittings 128, 130. The fittings 128, 130 connect the sample controlvalves 124, 126 to the first TDT 160 and the second TDT 162,respectively. Specifically, in the illustrated embodiment, the fittings128, 130 include nubbed, annular rings that allow them to be unscrewedfrom the first TDT 160 and the second TDT 162.

In one specific implementation, the TDTs just slip through holes 182,184 that have been bored through an aluminum plate/heat sink 152. Therecould be one TDT or two TDTs. In the illustrated embodiment, there isnothing holding TDTs 160, 162 in place except for the bottom fingernutconnection of the bottom fittings 132, 134.

In most cases the holes are vertical and the TDTs point up so the coolercan rest on the manifold.

Each of the first TDT 160 and the second TDT 162 are held in therespective through-holes 182, 184 that extend through the solidthermal/heat sink 152. In the illustrated embodiment, the thermal/heatsink 152 is fabricated from a solid block of aluminum. The through-holes182, 184 have been bored through that solid block and receive therespective first TDT 160 and the second TDT 162.

The bottoms of each of these TDT's 160, 162 connect respectively to afirst bottom fitting 132 and a second bottom fitting 134. Like the topfittings 128, 130, the bottom fittings 132, 134 include annular ringsthat allow them to be unscrewed from the first TDT 160 and the secondTDT 162.

The first bottom fitting 132 and the second bottom fitting 134 connectto a respective first vacuum port 136 and a second vacuum port 138.These vacuum ports are in fluid communication with a vacuum pump 114.Specifically, the vacuum pump is used to draw gas from the stream,through the manifold and the first TDT 160 and the second TDT 162 andthen the gas is exhausted through exhaust port 116.

The temperature of the thermal heat sink 152 is controlled by atemperature control module 150. In the preferred embodiment, thetemperature control module 150 is a thermoelectric cooler. In general,thermoelectric coolers use the Peltier effect to create a heat fluxbetween the junction of two different types of materials within thecoolers. These are solid-state devices that can function in a coolingmode or a heating mode to transfer heat from one side of the device tothe other, with consumption of electrical energy, depending on thedirection of the current. In some implementations, the Peltier coolingtechniques described herein are employed to set an ideal temperature forsample collection (e.g., siloxanes from biogas) or, if desired, thetemperature can be simply set just above 0° C., the freezing point ofwater. Other implementations rely on the thermoelectric cooler to heatand desorb the collected materials, in the process of analyzing thesample, by GC-FTIR, for example.

In the preferred embodiment, a temperature detector or sensor 186 issecured to the thermal/heat sink 152 or possibly to the thermoelectriccooler 150. This is used to detect the temperature at which the TDT's160, 162 are being held and the information is provided to thecontroller 11, which then controls the operation of the thermoelectriccooler 150 in a feedback loop in order to maintain a desired setpointtemperature.

Some thermoelectric coolers operate without feedback. These devices whenenergized can go to a specific Delta T from ambient. Such a system wouldbe much cheaper for home air testing or ambient air testing where theactual temperature is not as critical as say compliance emission testingwhere it is important to detect all possible compounds.

In one embodiment, a fan 170 is provided on the backside of thethermoelectric cooler 150 to allow for the efficient removal orcollection of heat depending on the mode of operation.

FIG. 2 shows an exemplary analysis system 10, including a separator 50for separating a sample, collected on the TDTs 160, 162, into itscomponents (e.g., separate compounds), a detector 58, such as aspectrometric system 60, for detecting the spectral response of thosecompounds in a sample cell 95, and a controller 11 that controls thesystem and uses the spectral information to identify the compounds ofthe sample and their concentrations.

The spectrometric system 60 can determine the spectral response of thecompounds in the sample cell 95 in one or more of the following spectralregions: near-, mid- and/or far-infrared, visible, and/or ultraviolet(UV) (including vacuum ultraviolet (VUV)). Further, the spectrometricsystem can measure different characteristics, such as absorptionspectra, emission (including blackbody or fluorescence) spectra, elasticscattering and reflection spectra, and/or inelastic scattering (e.g.,Raman and Compton scattering) spectra of the compounds in the samplecell.

In the case of optical spectrometric systems, for example, differenttechnologies can be employed. In Fourier transform infrared spectrometry(FTIR) systems, single beam spectra are generated by taking the rawinterferograms from the FTIR spectrometer and then converting thoseinterferograms to intensity versus wavenumber spectra. In othersituations, spectra might be directly read-out as in the case where thespectrometric system 60 is a post dispersive system, which includes abroadband source and a spectrally resolving detector system. In otherexamples, the spectrometric system 60 includes a tunable optical source(e.g., tunable laser) and a detector. Here, the spectral information isa function of the time response of the detector, in such apre-dispersive system.

Further, other detectors 58 such as a mass spectrometers could be usedin place of optical spectrometer.

In general, the spectrometric system 60 is preferably sufficientlysensitive so that by analysis of the spectral information, thecontroller 11 can detect at least some of the sample compounds with lowconcentration, such as in a few percent to low parts per million (ppm)concentrations, or lower, to parts per billion (ppb).

In a specific embodiment, the spectrometric system 60 is a FTIR system.Its sample cell 95, also referred to as “gas cell” 95, is provided withan inlet port 96 for receiving a separator line 90. The sample cell 95of the spectrometric system 60 has an outlet port 98 for venting thesample cell contents through an exit line 92. An exit valve 94 sealsand/or controls the flow from the sample cell 95. A vacuum pump 97 canbe provided after the exit valve 94 so that a vacuum or partial vacuumcan be drawn on the sample cell 95.

The sample cell 95 can have windows made of ZnSe, KBr, BaF₂ or CaF₂, forexample, and is fabricated from a suitable material, for instance weldedstainless steel. The cell can be configured for multiple-path (alsoknown as multiple-pass or long path) absorption. By increasing the pathlength traveled with respect to volume, multiple-pass arrangements canbe used to measure low concentration components or to observe weakabsorption spectral features without increasing the physical length orvolume of the cell itself. Since the detection limit of the system isdirectly related to the volume/path length ratio, decreasing the volumeor increasing the path length lowers the concentrations that can bedetected. Assuming no signal losses, doubling the path length orreducing the volume in half will lower the detection limit by a factorof 2.

In certain embodiments, longer path lengths are used in combination withhigher reflective coatings like enhanced silver, yielding a reflectivityin the 0.992 to 0.995 range or greater. Coating optimizations, in the IRregion, for example, could further improve reflectivity. This allows forpath lengths that are longer by a factor of 4 to 8 or even more.

Specific implementations utilize a sample cell 95 that is configured asa “White cell” type. The principles of a traditional White cellarrangement employ three spherical concave mirrors having the sameradius of curvature. These principles can be modified, to improve imagequality and optical throughput, as described, for instance, by Spartz etal. in U.S. Patent Application Publication No. 2015/0260695 A1 (now U.S.Pat. No. 9,606,088). In one example, the White cell type employed usesnon-spherical concave mirrors cut onto a single metal or a glass blank,providing a fixed path length; the mirrors can be the solid end caps ofthe sample cell, allowing for smaller sample cells that are easier toalign.

Other multiple pass cell designs that can be utilized include but arenot limited to Herriott cells, Pfund cells, cavity-ring down cells, andintegrating spheres.

In further examples, the sample cell 95 is a lightpipe flow throughsample cell.

The sample components are separated in time by the separation system 50,which is preferably a gas chromatography system. The GC system has a gaschromatographic column 48. Often the column 48 is coiled in order tominimize overall size while maintaining sufficient tube or columnlength. Column 48 has a proximate end or inlet 40 for receiving samplefrom a sample inlet line 88 and a distal end or outlet 52 for directingresulting product through the line 90 to the sample cell 95 for analysisin the spectrometry system 60.

The column 48 is typically held within a temperature controlled chamber44 with a heat source (e.g., an oven), such as a heating coil that isthermostatically controlled by the controller 11 in order to maintain aselected constant temperature during a gas chromatography analysis run.Typically, the heat source also provides sufficient heat to the chamberinterior so that the temperature is sufficiently high to ensure that thesample reaches a gaseous state. In one implementation, the column 48 isresistively heated, thus avoiding the need for the oven. Specifically,column 48 is heated directly by passing a current through the metalcolumn and monitoring the resistance to determine the temperature.

This system uses the TDTs 160, 162 as a concentrator. Further, if thesample contains trace concentrations, for example in the ppb or partsper trillion (ppt) range, a series of concentrators can be used in theanalysis system 10. Such configurations allow the same system to be usedfor a wide variety of samples and sampling conditions.

In one mode of operation, the vacuum pump 97 draws a vacuum on the gascell 95 and then the exit valve 94 is shut. In this mode, the cell 95integrates and collects compounds of a sample for a certain time period.

In another mode of operation, the vacuum pump 97 draws the samplethrough a flow gas cell 95 and then the exit valve 94 is shut.

The system 10 further includes an input director switching system 20 anda GC director switching system 30 for controlling the flow of gases intoand out of the TDT 160, 162 and GC 50.

The input director switching system 20 connects to a carrier gas source12, such as nitrogen, helium or other essentially inert gas that willnot interfere with detecting pollutants and other impurities. A massflow controller (MFC) 14 is preferably provided in-line between thecarrier gas source 12 and the input director 20 to control the flow rateof the carrier gas. Rotometers or other suitable flow regulators can beemployed. The input director switching system 20 then flows the carriergas through the TDT 24.

The GC director switching system 30 is connected for receiving sampleand carrier gas from the TDT 160, 162. Output from the GC directorswitching system 30 then provides gas to GC 50. Possibly a compressor 34may be provided inline between the GC director switching system 30 andthe GC 50.

In practice, the functions of the controller 11 are often distributedamong multiple computer systems. For example, one or two computersystems will often perform the functions of real-time control of thesystem 10 and the TDT collection system 100, and collecting and loggingthe data from the systems 10, 100. This includes controlling the flow ofgases and liquids throughout the system 10 by controlling one or moreMFCs, e.g., MFC 14, input director 20, GC director 30, collection anddesorption of TDT 160, 162, valves, e.g., exit valve 94, compressor 34,vacuum pump 96, and separator 50 in addition to the other components ofthe system 10. The real-time control functions further includecollecting and recording the spectral information from the spectrometricsystem 60. Then, an additional computer system will often be utilized toanalyze that data and identify the specific compounds of the sample.This includes analyzing the spectral information and how thatinformation changes over time and recording and reporting thecomponents/compounds present with their concentrations or mass to anoperator via a user interface or to another computer. These data arecompared with known preset amounts of concentrations (e.g., determinedin a calibration procedure) that the spectrometric system 60 is capableof detecting.

In one example, the TDT collection system 100 is inserted directly intothe analysis system 10. Then, under the control of the controller 11,the TDT 160, 162 are cooled and gas flowed through the tubes together orseparately. At the same time, the controller 11 controls the inputdirector switching system 20 to flow gas from the sample source 5 to thecollection system 100 and operates the thermoelectric cooler 150. Gasflowing through tube 118 is vented by the GC director switching system30 or coming through the TDTs is vented by input director switchingsystem 20.

After the sample has been collected in the tubes 160, 162, thecontroller 11 reverses the polarity applied to the TE cooler 150 so thatit heats the TDT 160, 162, together or separately to desorb thecollected samples directly into the separator/GC. The sample in thesample cell 95 is then analyzed by the spectrometric system 60. In thismode, the input director switching system 20 flows carrier gas such asnitrogen from the carrier gas source under the control of the mass flowcontroller 14 through the TDT 160, 162 and through the GC directorswitching system 30 to the GC 50.

For some applications, the GC 50 can be omitted or at least bypassed. Incases in which the FTIR analyzer is part of a GC-FTIR system (such asthat described in U.S. Pat. No. 9,606,088, to Spartz et al., forexample), the GC 50 can be bypassed and the sample from the TDT can beintroduced directly to the gas cell in the FTIR analyzer. In this mode,the GC director switching system 30 bypasses the GC 50 and sends thesample directly to the sample cell 95 using bypass line 144.

Arrangements that omit or bypass the separator (GC 50, for instance),can be particularly useful in the detection of non-volatile, often verynon-volatile, or low semi-volatile impurities in a gas. Oneimplementation relates to the detection and, often, the quantificationof oil contamination present in a biogas sample. Often this oil is tracepump or compressor oil, etc. As known in the art, the term “biogas” or“biomethane” refers to any gas fuel derived from the decay of organicmatter, e.g., a mixture of methane and carbon dioxide produced by thebacterial decomposition of sewage, manure, garbage, or plant crops.

The system described herein can be designed to handle the high biogasvolumes needed for collecting a sample sufficiently large for ppt (partsper trillion) or pg/L (picograms per liter) MDLs detection. Asensitivity of 20 ng, for example, may require a 1,000 L of biomethaneto reach 20 pg/L. Thus, detecting MDLs in the pg/L range can involvepassing very large amounts of biogas through the TDT.

Operating with such large volumes is facilitated by approaches thatcollect heavy impurities preferentially or exclusively. In oneembodiment, the TDT stationary phase utilizes a material thatpreferentially or exclusively collects non-volatile compounds such asoil. For example, the material can be a sorbent designed to collect onlyheavy components. It is also possible to use a material that may not bea sorbent in the true sense of the word but rather provide a surfacearea onto which heavy components (e.g., oil or other non- orsemi-volatile organic compounds) can stick.

In applications targeting biogas, the material also can be of a typethat does not trap (or lets pass through) volatile organic compounds(VOCs) and/or water. In other cases, the TDT material is held at atemperature that is sufficiently high to reduce or minimize trapping,sorbing, binding or otherwise collecting VOCs and/or water onto thematerial.

Some specific examples of TDT materials that can be employed includeglass wool, glass beads, metal frit and the like. Using such a materialwould allow operating at very large volumes and high flow rates tocollect more sample at a higher rate.

Oil collected on the TDT stationary phase can be desorbed or otherwisereleased directly to the FTIR analyzer. For example, the TDT is rapidlyheated (using a suitable heating device, e.g., heating tape, heatingjacket, oven, Peltier heater, cartridge heater, immersion heater, etc.)to vaporize the oil and direct the oil vapors, typically in a carriergas, via a heated transfer line to the FTIR analyzer. In oneimplementation, the TDT is heated by the thermoelectric cooler (e.g.,thermoelectric cooler 150) described herein.

FIG. 3 shows a TDT (thermal desorption tube) collection system 100,according to another embodiment.

Here, a TDT 160 is held upright in a through-hole 182 that extendsthrough a solid thermal/heat sink 152. This heat sink 152 may include aninsulation layer, not shown, to provide insulation from the surroundingenvironment to enable better temperature control. This thermal/heat sink152 can also be fabricated from a solid block of aluminum. Thethrough-hole 182 has been bored through that solid block.

The heat sink 152 is supported on a lower housing 190. The lower housing190 has a lower fitting 132 provided on its top face. This fitting islocated underneath the through-hole 182 of the heat sink 152. In oneembodiment, the TDT 160 is inserted into the through-hole 182 of theheat sink 152. It is then press fit into the lower fitting 132 whichseals against the tubes outer surface.

The lower housing 190 contains a diaphragm pump 114, a controller 11,and a battery 194, in one embodiment.

The diaphragm pump 114 is connected to the lower fitting 132. It drawsair through the TDT 160 and then exhausts that air to the ambientenvironment. In this way, air is drawn through the TDT so that anyvolatiles chemicals can be retained in the sorbent within the tube 160.

As before, the temperature of the thermal heat sink 152 is controlled bya temperature control module 150. In the preferred embodiment, thetemperature control module 150 is a thermoelectric cooler. As before, atemperature detector 186 is secured to the thermal/heat sink 152 orpossibly to the thermoelectric cooler 150. This is used to detect thetemperature at which the TDT 160 is being held. This information isprovided to the controller 11, which then controls the operation of thethermoelectric cooler 150 in a feedback loop in order to maintain adesired setpoint temperature.

The illustrated embodiment described with reference to FIG. 3 isintended for ambient air testing such as in the home or factory. Thus,in one embodiment, the system is powered by an onboard battery 194. Thisallows it to be located for easy testing at any location even if a wallplug power is not available.

FIG. 4 shows an integrated collection and analysis system 10, which cancollect samples using TDT's in a ping-pong fashion. The two-channelapproach illustrated in this embodiment can switch or alternate betweencollection and desorption and can find applications for onlinesampler/desorber systems.

In more detail, gas, such as hot exhaust gas from a chemical processingsystem or energy generation system, or gas from occupational or ambientair monitoring is directed into a gas inlet 110. It then flows through asample gas tube 118 to a gas outlet 112.

Gas samples can be taken from the gas stream via sample port 120, whichconnects to an input director switching system 20. This example includestwo arcuate loops A, B. In the illustrated setting of the input director20, the input gas sample flows through arc A of the input director 20 toa first input tube 208 that carries the gas to a first TDT 160.

At the same time, arcuate loop B of the input director 20 connects to asecond input tube 208, which connects to a second TDT 162.

The first TDT 160 and the second TDT 162 are held in a ping-pongtemperature control system 220. In more detail, the temperature controlsystem 220 comprises a thermoelectric cooler 150. The control system 220further includes a first heat sink 152A and a second heat sink 152B, oneither side of the thermoelectric cooler 150.

In this configuration, the first heat sink 152A and the second heat sink152B are located on opposite sides of thermoelectric cooler 150. Thus,when the thermoelectric cooler 150 is operated, one of the heat sinks isbeing cooled while the other heat sink is being heated. Then, theoperation can be reversed by simply switching the polarity of thethermoelectric cooler 150.

The temperatures of the heat sink 152A and 152B are monitored byseparate temperature detectors 186A, 186B that are secured to thethermal/heat sink 152 or possibly to the thermoelectric cooler 150. Theyare used to detect the temperature at which the TDTs 160, 162 are beingheld. This information is provided to the controller 11, which thencontrols the operation of the thermoelectric cooler 150 in a feedbackloop in order to maintain a desired setpoint temperature.

Each of the TDT's 160, 162 connects to respective output lines 212, 214.These output lines couple the TDT's 160, 162 to an output director 30.This output director similarly has an arcuate loop A and arcuate loop B.

The output director 30 selectively couples the output lines 212, 214 toeither a vacuum pump 114 or a gas chromatography system or separator 50.The gas chromatography (GC) system 50 in turn is coupled to a detector58. In one example, this detector 58 is a spectrometric system 60 with asample cell 95 as described earlier. In some embodiments, the GC isomitted/bypassed and output 212, 214 are passed by director 30 directlyto detector 58.

Finally, the controller 11 controls the operation of the GC 50, detector58, both the input director 20 and the output director 30. It furthercontrols the operation of the thermoelectric cooler 150 via athermoelectric driver 172.

With the input director 20 and the output director 30 set asillustrated, the first TDT 160 is connected to the vacuum pump 114. Itthus draws in the gas to be sampled from the sample gas tube 118 throughthe first TDT 160. Further, the controller 11 via the thermoelectricdriver 172 drives the thermoelectric cooler 150 so that the first heatsink 152A is being cooled. Thus, volatiles in the sample gas are trappedin the sorbent in the first TDT 160.

On the other hand, the thermoelectric cooler 150 heats the second heatsink 152B. The arcuate tube B of the input director is connected so thatthe second TDT 162 is connected to the carrier gas source 12. In someembodiments, a mass flow controller controls the rate at which thecarrier gas 12 flows into the TDT 162.

Since the second TDT 162 is being heated, the volatiles desorb from theTDT 162 and flow through the second output line 214 through the outputdirector 30 to the gas chromatography system or separator 50. Fromthere, the peaks eluting from the GC flow into the detector 58 foranalysis.

Then, the input director 20 and the output director 30 are preferablyswitched so that the first TDT 160 is placed in desorption mode whilethe second TDT 162 is placed in sampling mode.

In more detail, when the input director 20 is switched, the input gassample flows through arc A of the input director 20 to a second inputtube 210 that carries the gas to a second TDT 162.

At the same time, arcuate loop B of the input director 20 connects tothe input tube 210, which connects to a second TDT 162, to the carriergas source 12.

The output director 30 is also switched. Arcuate loop A now connects thesecond output line 214 to the vacuum pump 114, and arcuate loop Bconnects the first output line 212 to the separator or GC 50.

With the input director 20 and the output director 30 in the newsetting, the second TDT 162 is connected to the vacuum pump 114. It thusdraws in the gas to be sampled from the sample gas tube 118 through thesecond TDT 162. Further, the controller 11 via the thermoelectric driver172 switches the operation of the thermoelectric cooler 150 so that thesecond heat sink 152A is being cooled. Thus, volatiles are trapped inthe sorbent in the second TDT 160.

At the same time, the thermoelectric cooler heats the first heat sink152A. The arcuate tube B of the input director 20 is connected so thatthe first TDT 160 is connected to the carrier gas source 12. In someembodiments, the mass flow controller controls the rate at which thecarrier gas 12 flows into the first TDT 160.

Since the first TDT 160 is being heated, the volatiles desorb from theTDT 160 and flow through the first output line 214 through arcuate tubeB of the output director 30 to the gas chromatography system orseparator 50. From there, the peaks eluting from the GC flow into thedetector 58 for analysis.

One embodiment includes a separator 50 for separating a sample,collected on the TDTs 160, 162, into its components (e.g., separatecompounds), a detector 58, such as a spectrometric system 60, fordetecting the spectral response of those compounds in a sample cell 95,and a controller 11 that controls the system and uses the spectralinformation to identify the compounds of the sample and theirconcentrations.

Flow through the TDTs can be driven and controlled using one or moresuitable devices. Shown in FIG. 5, for example, is arrangement 300 inwhich TDT 160 and TDT 162, essentially as described above, are connectedto vacuum pump 114. As before, sample gas is introduced through samplegas tube 118. Flow from TDTs 160 and 162 (via conduits 304 and 306) iscontrolled, respectively, by mass flow controllers (MFCs) 308 and 310.In one example, the MFCs used are elastomer sealed digital mass flowcontrollers, e.g., MKS GE50A. Rotometers or other suitable devices alsocan be utilized. The arrangement also includes vacuum relief valve 312.

In still a different mode of operation, the system 100 is operated in“hot mode.” Such a mode could be used when collecting semi volatilecompounds, for example. In this mode of operation, the thermoelectriccooler 150 would heat one or both of these thermal desorption tubes 160,162 to a temperature that would be high enough such that they did notcollect water. For example, in one embodiment, the tubes are held at atemperature higher than 100° C. At this temperature, potentially muchlarger volumes could be sampled.

In further embodiments, the equipment and techniques described hereincan be applied or adapted to biogas analysis.

High quality biogas is required in some applications. For compressionengines, for example, impurities can harm the gas distributing system orthe gas utilities or cause unwanted exhaust products. Thus, it isimportant to detect biogas contaminants, even when present in very lowconcentrations. One such impurity is oil.

Aspects of the present invention present an approach for handling verylarge biogas samples to detect the presence of oil (e.g., pump orcompressor oil, derived, for instance, from the engine itself) at levelsas low as 10 to 20 nanograms (ng).

A method that can be used comprises collecting oil in a samplecollecting device, the thermal desorption tube (TDT) 160, 162, forexample, releasing the sample directly to a gas cell 95 of the FTIRanalyzer and obtaining a spectral response of the oil present in thecell. A system for analyzing oil in biomethane comprises at least onesample collecting device, a TDT, for example, and an FTIR analyzer. TheFTIR analyzer includes the sample cell 95, e.g., as described above, forreceiving the sample released from the sample collecting device. In someimplementations, the TDT is part of the thermal desorption tubecollection system described above.

In one embodiment, oil is collected on a first TDT (device 160 in FIG.5, for example) previously purged and ready to receive a new sample. Inthe meantime, oil trapped by the second TDT (e.g., device 162 in FIG.5), is released (e.g., by heated N2 carrier gas from source 12) from thestationary phase of the TDT and directed to the FTIR analyzer. Once allthe oil has been released from TDT 162, the second TDT 162 can be sweptby clean and preferably heated N2 gas in preparation for receiving a newsample. The first TDT (device 160) can now be heated to release thetrapped oil and direct it to the FTIR analyzer. Alternating theoperation of the two TDTs (devices 160 and 162) can double the flow rateto the FTIR analyzer (to 2 times 25 mL/min, for example). In contrast toarrangements in which a second, focusing, TDT is employed as a secondtrap downstream of the first, the ping pong arrangement of the two TDTsalternates the sample collection operation between the two collectiondevices (TDT 160 and TDT 162).

Principles described herein also can be used or adapted to detect andoften quantify the presence of herbicides, pesticides and the like inambient air.

Furthermore, it may be possible to conduct some aspects of the inventionusing spectral response of the analyte, e.g., oil, in sample cell 95 inother spectral regions e.g., visible, and/or ultraviolet (UV) (includingvacuum ultraviolet (VUV)). Further, the spectrometric system can measuredifferent characteristics, such as absorption spectra, emission(including blackbody or fluorescence) spectra, elastic scattering andreflection spectra, and/or inelastic scattering (e.g., Raman and Comptonscattering) spectra of the compounds in the sample cell.

The invention is further illustrated by the following non-limitingexamples.

Example 1—Cold Sampling of Acrolein, Acetaldehyde and Formaldehyde on A2TDTs

The purpose of this experiment was to determine a sampling temperaturethat yields >50% recovery for acrolein, acetaldehyde and formaldehyde onA2 TDTs (Gerstel). A 191° C. sample stream consisting of 2% moisture,acrolein, acetaldehyde and formaldehyde in nitrogen was run to thesampling manifold. A 2 L sample was collected on a pair of A2 TDTsloaded into the TDT collection system 100 at a rate of 100 mL/min for 20min, with one TDT at room temperature and one in a 0° C. container.Sampling was repeated twice more for a total of three consecutive samplepairs (Sample 6, 7 and 8). After sampling was complete, TDTs wereimmediately transferred to the refrigerator, where they were stored for10-30 minutes. TDTs were run on GC and analyzed using absorbance spectraand manual validation. Results for Sample 7 are not shown below becausethe run data for the 0° C. TDT was not useable. The

A2 TDTs Sample 6 Sample 8 Average 23° C. 0° C. 23° C. 0° C. 23° C. 0° C.Mass Percent Mass Percent Mass Mass Percent Mass Percent Mass Percent(ng) Recovery (ng) Recovery (ng) Recovery (ng) Recovery (ng) Recovery(ng) Recovery Acrolein 2010 39.90% 3230 64.11% 1910 37.91% 3260 64.71%1697 33.68% 3245 64.41% Acetaldehyde 815 40.75% 1440 72.00% 961 48.05%1430 71.50% 855 42.77% 1435 71.75% Formaldehyde 663 33.15% 1480 74.00%600 30.00% 1345 67.25% 798 39.88% 1412.5 70.63%

The table below shows results from two TDTs spiked with a BTEX andstyrene standard, dry purged, and run using either method, where VIAQ isthe known abbreviation for “Vehicle Indoor Air Quality”. Percentrecoveries are comparable between the two instrument methods. It isimportant to note that the standard has been in use for 3 weeks, whichis why recoveries are <90%. Benzene and toluene may be lower due toevaporation off the glass wool when the tube was being spiked with theliquid.

BTEXS with VIAQ Inst Method BTEXSN VIAQ Method Mass Percent Mass Percent(ng) Recovery (ng) Recovery Benzene 1380 66.61% 1190 57.44% Toluene 169082.71% 1410 69.01% Ethylbenzene 1870 91.16% 1600 78.00% m-Xylene 174083.57% 1590 76.37% p-Xylene 1810 88.12% 1760 85.69% o-Xylene 1780 87.38%1850 90.82% Styrene 1760 89.57% 2000 101.79%

Example 2—Biogas Analysis

Since, in many instances, the biogas analysis will not require adetailed breakdown to the level of each and every oil species present, areading for an entire class is sufficient. Thus, in someimplementations, the system and method described above can be used tomeasure oil as a single component at trace amounts of 10 to 20 ng, forexample.

The IR spectra can be analyzed by a suitable technique. Some embodimentsrely on calibrations based on decane, undecane, dodecane spectra todetermine the mass level of oil present (in ng). Dodecane, for example,is commonly used to match heavy hydrocarbons like diesel fuel and oil.Techniques described in U.S. patent application Ser. No. 16/113,856,filed on Aug. 27, 2018, with the title Carbon Ladder Calibration,incorporated herein by this reference in its entirety, can be used topredict which compound is most similar to the sample and use thiscompound to obtain the most precise reading of oil mass. It is expectedthat the ratio of CH₂ to CH₃ will determine the best match for the oil.For instance, a compound such as decane has a 4 to 1 ratio of CH₂ toCH₃, whereas dodecane has a 5 to 1 ratio.

FIGS. 6-13 are illustrative plots obtained in the analysis of a sampleoil in biogas, with FIG. 6 showing the FTIR spectrum of directvaporization of oil into a nitrogen gas (N₂) stream. Dodecane iscommonly utilized to match heavy hydrocarbons like diesel fuel and oil.Examples that use dodecane calibrations to detect and quantify thesample oil are presented in the plots (absorbance as a function ofwavefunction) of FIGS. 7 and 8. A sample oil versus pentadecanecalibration is shown in the absorbance as a function of wavenumber plotof FIG. 9. The results indicate that the CH₂ to CH₃ ratio in the sampleoil is significantly different from the CH₂ to CH₃ ratio in pentadecane.Decane, on the other hand, provides a very close match of the CH₂ to CH₃ratio in the sample oil, as shown in FIG. 10. Shown in FIG. 11 is anabsorbance versus wavenumber plot of the C—C region of an oil versusdecane calibration. The plot indicates a quanted value that is closer tothe expected concentration. The absorbance as a function of wavelengthplot of FIG. 12 is based on an octane calibration of the oil. Althoughoctane has a higher CH₃ to CH₂ ratio, it still provides a similarconcentration as the other calibration compounds. FIG. 13 provides alinear calibration curve for decane at levels from 0 to 800,000 ng,showing a constant calibration.

Some of the experiments conducted using the approach described aboveindicated that the oil appeared to resemble decane at 4 to 1. Othersuitable methods or software packages can be employed to suggest orpinpoint the most likely oil components present in the sample.

Once the oil mass value is determined it can be divided by the originalsample volume to calculate the pg/L or oil present in the biogas. Thisresult can be reported to a process distributed control system (DCS) atthe plant.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A sample analysis system comprising: two thermaldesorption tubes configured for a sample collection and sample analysisping pong arrangement; at least one heat sink having at least twothrough holes in which the two thermal desorption tubes are located; athermoelectric cooler for cooling a sample collection thermal desorptiontube while heating another sample analysis thermal desorption tube bycontrolling a temperature of the heat sink; a gas chromatography systemfor separating a vapor desorbed from the heated thermal desorption tubeinto components; and a sample cell for receiving the separatedcomponents from the gas chromatography system and electromagneticradiation for a spectrometric analysis of compounds in the sample cell.2. The sample analysis system of claim 1, wherein the spectrometricanalysis is FTIR spectrometry.
 3. The sample analysis system of claim 1,further comprising a controller.
 4. The sample analysis system of claim1, further comprising a first heat sink and a second heat sink disposedat opposite sides of the thermoelectric.
 5. The sample analysis systemof claim 1, wherein an input director system selectively couples inputsof the thermal desorption tubes to either a gas to be analyzed or acarrier gas.
 6. The sample analysis system of claim 1, wherein an outputdirector system selectively couples outputs from the thermal desorptiontubes to either the gas chromatography system or a vacuum pump.